1. Field of the Invention
The present invention relates to a ventilator designated to provide assisted breathing to a patient and is of the type having a gas flow generator, a pressure meter, a flow meter, and a control unit which determines a gas pressure on the basis of a preset tidal volume for the patient and measurement signals from the pressure meter and the flow meter, for each breathing cycle, and which regulates the gas flow generator to generate the determined gas pressure.
2. Description of the Prior Art
Assisted breathing is provided for patients retaining at least some ability to breathe spontaneously. Ideally, assisted breathing should eliminate needless breathing effort by the patient, be adaptable to any changes in the patient""s condition, contribute to the provision of good ventilation, especially in respect to the CO2 level, and remain stable, despite any changes in the interface between the patient and the ventilator.
The resistance and elasticity of tubing and the patient""s airways (including the lungs) all affect assisted breathing and the patient""s own breathing efforts. Since these parameters change, e.g. when the patient shift/position, when the patient""s condition improves or worsens etc., assisted breathing cannot be regarded as a constant function of the gas supplied in each breath.
Volume support breathing (VS) is one respiratory mode developed to supply assisted breathing. It is described, in principle, in FIG. 1. In VS the physician sets a target tidal volume to be achieved in each breath. When a spontaneous breath is detected, the ventilator imposes a first inspiration 2 of breathing gas, at a first positive pressure 4, on the patient. The volume of inspired gas is determined by measuring the flow 6 in the first inspiration 2 and integrating it over time. If the determined volume of gas does not correspond to the preset tidal volume, the next inspiration 8 is imposed at a second positive pressure 10 higher than in the first inspiration 2. The inspired volume is determined by measuring the flow 12. A third inspiration 14 is imposed at a third, even higher, positive pressure 16, and measurement of flow 18 will show that the inspired volume exceeds the target volume (in this example). Then a fourth inspiration 20 is imposed at a fourth positive pressure 22 lower than the third positive pressure 16. The resulting flow 24 produces a tidal volume in agreement With the preset value.
Pressure limitations, an apnea alarm, switching to the controlled mode if apnea occurs etc. are other functions in this operating mode, which contribute to good patient safety. Even though this mode is effective and is frequently used by physicians, it still has room for improvement, especially since it does not take into account nor adapt in an optimal fashion to, e.g. changes in airway and equipment elasticity and resistance.
Proportional assisted ventilation (PAV) is another breathing mode. This mode strives to provide assisted breathing adapted to the patient""s breathing. In simple terms, this mode can be said to provide breathing assistance proportional to the patient""s attempts at breathing. The proportion can be e.g. 1:3. In PAV, the patient""s elastance is viewed as a constant, and the tidal volume varies with (and is related to) the patient""s efforts.
The main disadvantages of PAV are that changes in the patient""s condition are not taken into account, and the regulatory system is based on positive feedback, thereby making the system unstable. Therefore, this mode demands almost constant attendance of a doctor by the ventilator to adjust ventilator settings.
Automatic tube compensation (ATC) is a third known breathing mode. This is actually not a mode in itself but a way to compensate for the breathing resistance that develops in a tracheal tube with which the patient is connected to the ventilator. When compensation is provided for the tracheal tube""s resistance, the patient should be able to breathe as if no tracheal tube were present. (For a layman, spontaneous breathing through a tracheal tube can be likened to breathing through a straw. Such breathing is very hard on a patient with a diseased or damaged lung.)
None of the breathing modes provides simultaneous adaptation to variations in the patient""s intrinsic breathing or a predictable level of CO2.
An object of the present invention is to provide a ventilator capable of generating a breathing mode that resolves the aforementioned problems.
The above object is achieved in accordance with the principles of the present invention in a ventilator of the type initially described, wherein the control unit determines the gas pressure which is to be generated by the gas flow generator on the basis of mechanical resistances xe2x80x9cseenxe2x80x9d by the ventilator and/or on the basis of a variable corresponding to the aggregate effect of the resistance and elasticity of the lungs of the patient.
When appropriate parts of known breathing modes are combined and these are combined with new components, a breathing mode is achieved with the advantages of other breathing modes but without their disadvantages. The new breathing mode regards e.g. the patient""s elasticity and resistance as a variable (aggregated into a kind of impedance). As in VS, a physician sets a target for tidal volume, but the ventilator in the new breathing mode compensates for resistance in the ventilator system (primarily in the tracheal tube) and/or for variations in the patient""s xe2x80x9cimpedancexe2x80x9d, which can be a result of improvement in or worsening of the patient""s condition. The inventive breathing mode therefore can be referred to as xe2x80x9ccompensated volume supportxe2x80x9d (CVS).
In contrast to conventional volume support, VS, the pressure level can be controlled during the inspiration phase of CVS. Each breath will then result in a supplied tidal volume corresponding to the selected tidal volume.